c. 1400, scracchen, transitive, "mark or wound slightly on the surface by a scraping or tearing action with something rough, sharp, or pointed," probably a fusion of Middle English scratten and crachen, both meaning "to scratch," both of uncertain origin. Also compare scr-. Related: Scratched; scratching.
The meaning "relieve skin irritation by a scraping motion with the nails or claws or a scratcher" is by 1520s. The billiards sense of "hit the cue ball into a pocket" is recorded by 1909 (also, originally, itch), though earlier it meant "a lucky shot" (1850). The meaning "to withdraw (a horse) from a race" is 1865, from notion of scratching its name off a list of competitors; the phrase was used in a non-sporting sense of "cancel a plan, etc." by 1680s.
To scratch the surface "make only slight progress in penetrating or understanding" is from 1882. To scratch (one's) head as a gesture of perplexity is recorded from 1712. The plastering scratch-coat, roughened by scratching before it sets, is by 1891.
1580s, "a slight wound or laceration, slight tear in a skin or surface produced by something sharp or rough," from scratch (v.). Meaning "mark or slight furrow in metal, etc." is from 1660s.
The American English slang sense of "(paper) money" is from 1914, of uncertain signification. Many figurative senses (such as up to scratch, originally "ready to meet one's opponent") are from sporting use for "line or mark drawn as a starting place for contestants," attested from 1778 (but the earliest use is figurative). The meaning "nothing" (as in from scratch) is by 1918, generalized from specific 19c. sporting sense of "starting point of a competitor who receives no odds in a handicap match."
The use of the word in billiards is from 1850, originally "a stroke which is successful but not in the intended way." The meaning "pocket the cue ball" is by 1914. Scratch-pad, for hurried writing or drawing, is attested from 1883.
in Old Scratch "the Devil," 1740, a variant of Middle English Scrat, scratte "monster, goblin" (mid-13c. in place names), which is probably from Old Norse skratte "goblin, wizard," probably originally "monster" (compare Old High German scraz, scrato "satyr, wood demon," German Schratt, Old High German screz "a goblin, imp, dwarf"). It also was borrowed from Germanic into Slavic, as in Polish skrzat "a goblin." Old English forms of it were scritta, used to gloss Latin hermaphroditus, and scrætte "adulteress, harlot."